As is well known, transport aircraft travelling at typical cruise altitudes require pressurization of their cabins as well as temperature conditioning of the air to maintain the comfort of the occupants and, in many instances, to provide a proper temperature operating level for onboard equipment. It is also known that the vast majority of modern day aircraft requiring such systems include fuel consuming, gas turbine engines, both for propulsion and for use as auxiliary power units.
In conventional pressurization systems, bleed air is extracted from the main propulsion engines to provide both the cabin ventilation air and the energy for cabin pressurization. Bleed air exits the engine compressor section at an elevated temperature and pressure and conventionally is cooled with so called "ram air" before it enters a compressor known as a cabin compressor. The cabin compressor elevates the pressure to still a higher level and, of course, there is a concomitant increase in air temperature.
This air is then typically cooled in a second ram air heat exchanger and cooled once again in the reheater by condenser outlet air before flowing through a system condenser whereat the bleed air is cooled below its dew point by the exhaust stream of a turbine employed to drive the cabin compressor.
Once the condensed moisture is removed, the air from the condenser is returned to a reheater where it's temperature is increased prior to entering the turbine. Energy is extracted at the turbine to lower the air temperature and pressure while generating the power to drive the cabin compressor. The air from the turbine outlet has heat rejected to it at the condenser and then flows to a mixing plenum just prior to distribution to the cabin.
Such a system typically relies on the use of main engine bleed air (although it may use bleed air from an auxiliary power unit in some instances) to provide all of the fresh air required by the cabin. As a consequence, the bleed air penalty is substantial and is the most significant operating expense for certain types of aircrafts such as Boeing 757 and 767 aircraft.
Moreover, as new turbine engine designs achieve even higher and higher bypass ratios, the penalties for bleed air extraction are even greater than for present day engines due to the further reduction in engine core air flow that is available to be utilized in part as bleed air. Accordingly, reduced dependence upon bleed air is a major design issue for modern aircraft because significant cost savings in operation can be realized. Even small bleed air savings are thought to result in significant operational cost savings.
The present invention is directed to reducing the quantity of bleed air required to operate a pressurization and environmental control system in an aircraft to achieve significant operational cost savings.